WASHINGTON, D.C. -- One Metro transit train smashed into the rear of another at the height of Washington's Monday evening rush hour, killing at least four people and injuring scores of others as cars of the trailing train jackknifed into the air and fell atop the first.

District of Columbia fire spokesman Alan Etter said crews were cutting apart the trains to get people out in what he described as a "mass casualty event." Rescue workers propped steel ladders up to the upper train cars to help survivors escape. Seats from the smashed cars had spilled out onto the track.

D.C. Mayor Adrian Fenty said four were dead and many more hurt. Fire Chief Dennis Rubin said rescue workers had treated 70 people at the scene and sent some of them to local hospitals, two with life-threatening injuries. A Metro official the dead included the female operator of the trailing train. Her name was not immediately released.

The crash around 5 p.m. EDT took place on the system's red line, Metro's busiest, which runs below ground for much of its length but is at ground level at the site near the Maryland border.

Metro chief John Cato said the first train was stopped on the tracks, waiting for another to clear the station ahead, when the trailing train plowed into it from behind. Officials had no explanation for the accident.

Passenger Jodie Wickett, a nurse, told CNN she was seated on one train, sending text messages on her phone, when she felt the impact. She said she texted someone that it felt like the train had hit a bump.

"From that point on, it happened so fast, I flew out of the seat and hit my head." Wickett said she stayed at the scene and tried to help. She said "people are just in very bad shape."

"The people that were hurt, the ones that could speak, were calling back as we called out to them," she said. "Lots of people were upset and crying, but there were no screams."

One man said he was riding a bicycle across a bridge over the Metro tracks when the sound of the collision got his attention.

"I didn't see any panic," Barry Student said. "The whole situation was so surreal."

Homeland Security Department spokeswoman Amy Kudwa said less than two hours after the crash that federal authorities had no indication of any terrorism connection.

Metro general manager John Catoe said at least 60 people had been taken off the trains.

"I don't know the reason for this accident," he said. "I would still say the system is safe, but we've had an incident."

The only other time in Metrorail's 33-year history that there were customer fatalities was in January 1982, when three people died as a result of a derailment between the Federal Triangle and Smithsonian Metro stations underneath downtown.